The basic structure using RESURF (reduced surface field) principle consists of a low-doped P-type substrate and a low-doped N-type epitaxial layer. A P well is formed on the epitaxial layer and N+, P+ are implanted into the P well, such that a transverse P-well/N-epi junction and a longitudinal P-sub/N-epi junction are formed. Due to a higher doping concentration at both ends of the transverse junction, the breakdown voltage of the transverse junction is lower than that of the longitudinal junction. The basic principle of RESURF is to enable the epitaxial layer to be completely depleted before the transverse junction reaches the critical avalanche breakdown field by using the interaction of the transverse junction and the longitudinal junction. By reasonably optimizing the device parameters, the breakdown of the device occurs in the longitudinal junction, thereby playing a role in reducing the surface field.
In order to improve an on-resistance Rsp, the conventional laterally diffused metal-oxide-semiconductor field-effect transistor (LDMOSFET) having a RESURF structure mainly adjusts the impurity concentration of a drift region while satisfying the RESURF requirement. However, due to the inverse relationship between doping concentration and off-state breakdown, improving the on-resistance only by improving the resistance Rdr of the drift region has its limitations.